1. Field of the Invention
The present invention relates to oscillators, and more particularly to an oscillator including an oscillation circuit unit and an amplification circuit unit for amplifying an oscillation signal output from the oscillation circuit unit.
2. Description of the Related Art
FIG. 2 is a circuit diagram of a conventional oscillator. Referring to FIG. 2, an oscillation circuit unit 40 includes an oscillation transistor 41 and a resonance circuit 42. The collector of the oscillation transistor 41 is grounded via a ground capacitor 43 in the high-frequency range (i.e. at high frequencies), and the emitter thereof is grounded via an emitter bias resistor 44. Furthermore, capacitors 45 and 46 are connected for feedback purposes between the base and the emitter and between the emitter and the ground, respectively.
The resonance circuit 42 is implemented by a two-port circuit including a resonance line 42a implemented by a microstrip line, a varactor diode 42b, etc. One end of the resonance line 42a and the anode of the varactor diode 42b are grounded, and the other end of the resonance line 42a is connected to the base of the oscillation transistor 41 via a coupling capacitor 42c and to the cathode of the varactor diode 42b via a DC blocking capacitor 42d. Thus, the oscillation circuit unit 40 constitutes an oscillation circuit of the common-collector type.
The cathode of the varactor diode 42b is connected to a control terminal 48 via a choke inductor 47. The control terminal 48 is grounded via a ground capacitor 49 in the high-frequency range. A voltage for changing the capacitance of the varactor diode 42b is applied to the control terminal 48.
An amplification circuit unit 50 includes an amplification transistor 51, the emitter thereof being connected to the collector of the oscillation transistor 41, and the collector thereof being connected to a power supply terminal 53 via an inductor 52. A voltage Vb is applied to the power supply terminal 53. The power supply terminal 53 is grounded via a ground capacitor 54 in the high-frequency range. The collector is grounded via a capacitor 55, and connected to an output terminal 57 via an output matching circuit formed by the inductor 52 and a coupling capacitor 56.
In association with base bias resistors 58, 59, and 60 connected in series between the power supply terminal 53 and the ground, bias voltages are applied to the bases of the oscillation transistor 41 and the amplification transistor 51. Thus, a common bias current flows into the oscillation transistor 41 and the amplification transistor 51. Thus, the amplification circuit unit 50 constitutes an amplification circuit of the common-emitter type.
A coupling capacitor 61 is connected between the emitter of the oscillation transistor 41 and the base of the amplification transistor 51, so that an oscillation signal is input to the base of the amplification transistor 51.
The oscillation frequency changes according to the voltage applied to the control terminal 48.
In the above-described conventional oscillator, the emitter of the oscillation transistor 41 is connected to the base of the amplification transistor 51 via the coupling capacitor 61. Since the reactance of the coupling capacitor 61 depends on frequency, the level of the oscillation signal input to the base of the amplification transistor 51 is higher when the oscillation frequency is high and lower when the oscillation frequency is low.
Thus, the level of the oscillation signal output from the amplification circuit unit 50 also changes depending on the oscillation frequency. It would be desirable to provide an oscillator having a substantially constant level of oscillation signal over the entire range of oscillation frequencies.
Accordingly, an object of the present invention is to provide an oscillator in which the level of the oscillation signal is substantially constant over the entire oscillation frequency range.
To this end, the present invention provides an oscillator comprising an oscillation circuit unit having an oscillation transistor and a resonance circuit, the collector of the oscillation transistor being grounded via a first capacitor; and an amplification circuit unit of the common base type, having an amplification transistor, the emitter of the amplification transistor being directly connected to the collector of the oscillation transistor, and the base of the amplification transistor being grounded via a second capacitor. The resonance circuit is connected between the base of the oscillation transistor and the ground, an oscillation signal output from the collector of the oscillation transistor is input to the emitter of the amplification transistor while partially being bypassed to the ground via the first capacitor, and a negative feedback is provided to the amplification transistor in association with the second capacitor.
Thus, the level of the oscillation signal output from the amplification circuit unit is substantially constant even if the oscillation frequency changes.
Preferably, the reactance of each of the first capacitor and the second capacitor is in a range of a few ohms to dozens of ohms at the oscillation frequency.
Thus, the level of the oscillation signal-input to the emitter of the amplification transistor is maintained sufficiently high while a sufficient gain is provided in the amplification circuit unit.
More preferably, the reactance of either the first capacitor or the second capacitor is within three times the reactance of the other.
Thus, substantial flatness is achieved in the level of the oscillation signal over the oscillation frequency range.
More preferably, the reactance of the first capacitor is larger than the reactance of the second capacitor.
Thus, the level of higher harmonics input to the amplification circuit unit is decreased.
More preferably, the oscillation circuit unit oscillates in a range of approximately 3.5 GHz to 3.9 GHz, the capacitance of the first capacitor is approximately 2 pF, and the capacitance of the second capacitor is approximately 4 pF.
Thus, the level of the oscillation signal output from the oscillation circuit unit is substantially constant in the oscillation frequency range of 3.5 GHz to 3.9 GHz.